1. Field of the Invention
The present invention relates to a voltage controlled emitter-coupled multivibrator, and more particularly to a voltage controlled emitter-coupled multivibrator which can determine an oscillating frequency range and adjust the height of a frequency within the determined frequency range.
2. Description of the Prior Art
A voltage controlled oscillator is utilized in a phase locked loop (PLL) or a frequency combining apparatus, and particularly in the PLL for identifying the frequency of an outputting signal with a predetermined frequency. An example of the voltage controlled oscillator used in the PLL is disclosed in U.S. Pat. No. 4,749,961 (issued to Kato et al., on Jun. 7, 1988). Most voltage controlled oscillators make an oscillation by charging or discharging a capacitor through switching the amount of current controlled by an inputting voltage. The EC multivibrator is one of a relaxation oscillator using symmetric charging and discharging with respect to a timing capacitor connected between emitters of a differential gain stage.
FIG. 1 is a block diagram illustrating the constitution of the conventional EC multivibrator voltage controlled oscillator 10. In the conventional EC multivibrator voltage controlled oscillator 10, a gain stage 11 is comprised of a pair of first and second transistors Q1 and Q2. First and second transistors Q1 and Q2 are cross coupled through an emitter-follower buffer stage 12. First and second transistors Q1 and Q2 of gain stage 11 have equal load resistors R1 and R2 with clamping diodes D1 and D2, and are clamped to IV.sub.BE (=0.7) by clamping diodes D1 and D2. The emitters of first and second transistors Q1 and Q2 are biased by current sources I.sub.1 and are coupled through a timing capacitor C.sub.1.
The operation of the conventional EC multivibrator voltage controlled oscillator 10 having the above described constitution, will be explained.
Since first and second transistors Q1 and Q2 of gain stage 11 are combined by first and second transistors Q3 and Q4 of emitter follower buffer stage 12, either first or second transistor Q1 or Q2 (but not both) is on at any one time. In this manner, timing capacitor C.sub.1 is alternately charged with equal but opposite currents.
First, assuming for the moment that first transistor Q1 is off and second transistor Q2 is on, at the emitter of second transistor Q2 a total current of 2I flows, where one-half of the current is forced through timing capacitor C.sub.1. At this time, V.sub.B4 =V.sub.CC -V.sub.BE (FIG. 2A), V.sub.E4 =V.sub.B1 =V.sub.CC -V.sub.BE (FIG. 2D), and V.sub.B3 =V.sub.CC (FIG. 2B) when ignoring the base current of first transistor Q3, V.sub.E3 =V.sub.B2 =V.sub.CC -V.sub.BE (FIG. 2C) and V.sub.E2 =V.sub.CC -V.sub.BE (FIG. 2E).
Since first transistor Q1 is off, the current I charging timing capacitor C.sub.1 is obtained from the emitter of second transistor Q2. This current I causes the voltage level V.sub.E1 at the emitter of first transistor Q1 to decline at a constant slope of I/C.sub.1 until the voltage level at the emitter of first transistor Q1 becomes equal to V.sub.CC -3V.sub.BE, as illustrated in FIG. 2F. At this time, first transistor Q1 is on.
When first transistor Q1 is on, first diode D1 is on, V.sub.B3 =V.sub.CC -V.sub.BE (FIG. 2B), and V.sub.E3 =V.sub.B2 =V.sub.CC -2V.sub.BE (FIG. 2C) and second transistor Q2 is off. When second transistor Q2 is off, V.sub.B4 =V.sub.CC (FIG. 2A), V.sub.E4 =V.sub.B1 =V.sub.CC -V.sub.BE (FIG. 2D) and V.sub.E1 =V.sub.CC -2V.sub.BE (FIG. 2F). At this time, timing capacitor C.sub.1 discharges in the opposite direction with constant current I through second transistor Q2. When V.sub.E1 =V.sub.CC -3V.sub.BE (FIG. 2F), the oscillator is changed into its prior state to oscillate a constant frequency signal (V.sub.E2 -V.sub.E1 in FIG. 2G) through the operation.
However, in the above-mentioned conventional EC multivibrator voltage controlled oscillator, the oscillating frequency range cannot be established and the oscillating frequency cannot be adjusted.